The invention relates generally to the field of electronic photography, and in particular to a single sensor camera of the type that utilizes a color filter array.
A typical digital camera uses a single color image sensor (e.g., a charge coupled device (CCD)) to capture a representation of a scene, including a representation of the color in the scene. These sensor devices typically comprise a two-dimensional array of photosite pixels that measure the light at each point on the array. To allow these sensors to capture a color image, a color filter array (CFA) is placed above the pixels so that each pixel measures the light of the color of its associated filter. One common CFA pattern is known as the Bayer RGB pattern. As shown in FIG. 1, the Bayer CFA (which is also described in U.S. Pat. No. 3,971,065) provides a pattern of red, green and blue filters, including a checkerboard arrangement of the green filters. Therefore, the primary CFA image signal for each pixel relates to but one color, that is, to either red, green or blue (for a Bayer CFA). After the CFA image is captured, the remaining (missing) colors for each pixel site are interpolated for each pixel from, e.g., surrounding pixels, so that a full resolution interpolated record is generated for each image. Each pixel in the interpolated record therefore has a set of RGB values.
In some digital cameras, it is necessary to capture images at a resolution that is lower than the normal resolution of the CCD sensor. One reason for this is the need to take a series of pictures quickly, such as a short low resolution video burst, with minimal memory and image processing time requirements. To accomplish this, the pixels in the CCD sensor are often subsampled. For example, U.S. Pat. No. 5,493,335 describes a variety of subsampling patterns, including one pattern based on 2xc3x972 blocks of pixels. The CFA pattern in FIG. 2 shows how the pixels can be subsampled in 2xc3x972 blocks at half resolution. In this pattern as shown in the figure, the R, G, and B pixels represent sampled red, green, and blue pixels, while the dashes represent pixels that are not stored. In this configuration, the valid pixels can be read from the CCD hardware much more quickly and stored in a smaller memory area. The time required to process the resulting image is also smaller.
However, the problem with this approach is that the resulting image quality is low because of the irregularly sampled pixels. It is common for images to appear slightly blocky and for edges to appear wavy because of the sampling pattern. It is desirable to find a method of correcting for this type of sampling as a preprocessing step while still using the rest of the normal image processing hardware or software.
The present invention is directed to overcoming one or more of the problems set forth above. Briefly summarized, according to one aspect of the present invention, a method for generating color image signals derived from an array of color image pixels comprises the steps of: subsampling the array of color pixels in order to generate an array of subsampled color pixels; interpolating an array of new, evenly spaced color pixels from the subsampled color pixels using surrounding pixels of the same color; and processing the color image signals derived from the array of new color pixels.
The advantage of this invention is that it corrects for the blocky and wavy artifacts caused by an irregular sampling pattern. Moreover, these artifacts can be corrected in a preprocessing step while still using the rest of the normal image processing hardware or software on the resultant subsampled CFA image.
These and other aspects, objects, features and advantages of the present invention will be more clearly understood and appreciated from a review of the following detailed description of the preferred embodiments and appended claims, and by reference to the accompanying drawings.